The regulation of acetohydroxy acid synthase (AHAS) activity and synthesis is proposed here as a potential model system for the study of control of dispersed genes and their products. A valine-sensitive strain of Escherichia coli (K-12) was used to obtain mutations that result in AHAS activity which is resistant to feedback inhibition by valine (AHASr). Multiple, unlinked loci produce AHASr activity. These loci are all independent of the normal structural genes for AHAS isozymes. This study is proposed to analyze the products of the genes which, when mutated, produce a valine-resistant AHAS activity. Sucrose gradient analyses will be made to determine whether the mutated genes turn on ilvG, which is inactive in strain K-12 and codes for an AHASr. Genetic analyses will distinguish between the possibilities that the multiple loci are mutationally activated genes for AHASr or are genes for regulatory proteins that normally cause AHAS to be valine sensitive (AHASs). Bacteriophage Mu will be used to inactivate these genes to determine whether any are essential for expression of AHASr or AHASs. Each AHAS related gene will be introduced into strains that produce only one isozyme of AHAS to assess how these genes affect expression of each isozyme. The differential rate of synthesis of AHASr will be measured during growth of these strains under a variety of conditions, which include branched-chain amino acid limitation in a chemostat. Thus the roles of five genes (ilv-521, ilvF, ilvJ, ilv-660 and ilv661) in the regulation of synthesis and activity of AHAS isozymes will result from this study.